1. Field of the Invention
The present invention relates to technique for optical measurement, particularly relates to technique for measuring the information of the inside of a living body using light.
2. Description of the Related Art
Living body measuring technique using near infrared rays is applied to the measurement of a brain function. In Japanese Patent Application Laid-Open No. 9-098972, technique for simultaneously measuring a brain function at multiple points using rays of two wavelengths is disclosed and is used for measuring technique for imaging a brain function.
As shown in FIG. 2, the intensity change of transmitted light is measured by detecting light 2-1 radiated from the upside of a head skin 2-3 as detected light 2-2 at an apart point again. Based upon the change, the change of the concentration of hemoglobin in a brain cortex 2-5 between the radiated point and the detected point can be calculated. A reference number 2-4 in FIG. 2 denotes a skull and 2-6 denotes a measured middle part. Hemoglobin is classified into oxygenated hemoglobin and deoxidized hemoglobin depending upon an oxygenated state, however, as shown in FIG. 3, as respective absorption spectrums (absorption coefficients) 3-1, 3-2 are different, the change in the concentration of each hemoglobin can be independently measured by using rays of different two wavelengths. Heretofore, wavelengths of 780 nm (3-3) and 830 nm (3-4) have been often used.
An expression for calculating the change in each concentration of oxygenated hemoglobin and deoxidized hemoglobin is disclosed in Japanese Patent Application Laid-Open No. 9-098972 and on pages 1997 to 2005 of Medical Physics, 1995 No. 22 for example. As a blood oxidized state locally changes as a brain is activated, the change in each concentration of oxygenated hemoglobin and deoxidized hemoglobin is used for one of indexes showing the activity of a nerve.
It is disclosed on pages 1108 to 1114 of Medical Physics, 2001 No. 28 vol. 6 that in the method of measuring metabolism in a living body, the degree of an error varies depending upon a measuring wavelength. On the pages, an error of measurement in case a wavelength combined with 830 nm is shorter than conventional 780 nm in measurement in which distance (30 mm) between irradiation and detection is fixed is discussed. When a wavelength of 780 nm is made shorter in case it is supposed that the intensity of transmitted light and the magnitude of noise included in the intensity of transmitted light do not depend upon a wavelength, an absorption coefficient for deoxidized hemoglobin is increased and an error of the change in the concentration of hemoglobin in measurement is reduced. FIG. 4 shows the dependency theoretically shown of an error in measurement upon a wavelength. The x-axis shows the other measuring wavelength in case one measuring wavelength is fixed to 830 nm and the y-axis shows an error in measurement (the amplitude of noise). A reference number 4-3 denotes a wavelength of 780 nm often used heretofore. In FIG. 4, on the supposition that the magnitude of noise included in an original signal (a transmitted light signal) is fixed in each wavelength, an error in measurement (shown by a dotted line 4-1) of oxygenated hemoglobin and an error in measurement (shown by a full line 4-2) of deoxidized hemoglobin are shown. The validity of the theoretical prediction is verified in the measurement of a parietal region (the number of a living body=1).
As described above, a tendency that an error in measurement is reduced by using light of a shorter wavelength than 780 nm which has been often used in conventional type measurement equipment is known.
A method of selecting a wavelength suitable for measuring metabolism in a body is disclosed in Japanese Patent Application Laid-Open No. 7-222736 for example. In the patent application, a method of selecting a wavelength based upon a method of measuring not reflected light but light transmitted in a body and in consideration of the size of an object to be measured, that is, distance between irradiation and detection is proposed. For a condition to be a selection criterion, there are the following two conditions of a condition for precisely measuring an oxygenated state of hemoglobin and a condition for acquiring full transmitted luminous energy.
1) In case light of a wavelength in which difference between the absorbed amount of oxygenated hemoglobin and that of deoxidized hemoglobin is large is used, the change of an oxygenated state of hemoglobin can be precisely detected. Therefore, short wavelengths of approximately 600 nm are suitable.
2) To detect full transmitted luminous energy, a wavelength having high light transmittance in a body is required. Therefore, long wavelengths of the latter half of 700 nm to 900 nm are suitable.
As wavelengths that meet each condition described above are different, a method of selecting an optimum wavelength according to distance between irradiation and detection which is one cause which varies transmitted luminous energy in consideration of both conditions is disclosed in the above patent application.
In a method of measuring information inside a body using reflected light, measurement in the same depth is required and in addition, in case plural measurement points are set and imaging is required, a measuring method in which distance between irradiation and detection is fixed is adopted. Therefore, in prior art in which a wavelength was selected according to the variation of distance between irradiation and detection, a fixed measuring wavelength was always used.
However, as a result of measuring various regions, a wavelength suitable for reducing an error in measurement is different in regions different in a tissue even in measurement in case distance between irradiation and detection is the same. In case a shorter wavelength is used for a wavelength combined with 830 nm, an error in measurement is gradually reduced up to a wavelength of a certain value, however, an error increases from the wavelength of the certain value.
It is known that for example, the tissues of a body represented by a bone and a skin have different optical properties (an absorption coefficient and a light scattering coefficient). In the human head, the thickness of a bone, a skin and a muscle is different depending upon a region and an optical property is different every region. Therefore, a method of selecting a wavelength according to distance between irradiation and detection has a problem that a precise signal cannot be acquired.